This invention relates to a slip ring apparatus for an automotive alternator (i.e., alternating-current generator) installed in an engine room of a passenger car, a truck, or other automotive vehicles.
Conventionally, the automotive alternators are roughly classified into a first type which includes a slip ring directly formed at a protruding portion of a rotor shaft and a second type which includes an independent slip ring apparatus as a separate unit being press-fitted around the rotor shaft.
For example, FIG. 10 shows a conventional slip ring apparatus 101 serving as a separate unit. According to this slip ring apparatus 101, a first slip ring 111 and a second slip ring 112 are electrically insulated from each other and disposed coaxially with each other. The first slip ring 111 is connected via a first metallic lead 113 to one end of a field coil of the automotive alternator (positioned at the left side of the slip ring apparatus 101 although not shown in the drawing). The second slip ring 112 is connected via a second metallic lead 114 to the other end of the field coil of the automotive alternator. Both of the first metallic lead 113 and the second metallic lead 114 are insert molded in a resin or comparable insulating member 115.
The insulating member 115 includes a cylindrical body 115a with an outer cylindrical surface on which the slip rings 111 and 112 are fixed and an inner cylindrical surface 115e defining a hollow space. An annular terminal holding portion 115b, having a diameter greater than that of the cylindrical body 115a, is provided at the opposite side of the slip ring apparatus 101 far from the cylindrical body 115a on which the first and second slip rings 111 and 112 are provided. Two branch portions 115c and 115d are disposed in parallel with each other along the axis of the slip ring apparatus 101 (i.e., along the axis of the cylindrical body 115a) and are symmetrically opposed about an axial center of the slip ring apparatus 101. Each of the branch portions 115c and 115d extends straight in the axial direction so as to connect the cylindrical body 115a to the terminal holding portion 115b. 
The first metallic lead 113 extends axially in the cylindrical body 115a from the first slip ring 111 and then, at the portion embedded in the branch portion 115c, extends straight along the axis of the slip ring apparatus 101 (i.e., along the axis of the cylindrical body 115a). Then, the first metallic lead 113 bends perpendicularly in or near a merging region where the branch portion 115c changes into the terminal holding portion 115b, and further extends radially outward in the terminal holding portion 115b. A terminal end 113a of the first metallic lead 113 protrudes out of the annular terminal holding portion 115b and is connected to the one end of the field coil of the automotive alternator.
The second metallic lead 114 extends axially in the cylindrical body 115a from the second slip ring 112 and then, at the portion embedded in the branch portion 115d, extends straight along the axis of the slip ring apparatus 101 (i.e., along the axis of the cylindrical body 115a). Then, the second metallic lead 114 bends perpendicularly in or near a merging region where the branch portion 115d changes into the terminal holding portion 115b, and further extends radially outward in the terminal holding portion 115b. A terminal end 114a of the second metallic lead 114 protrudes out of the annular terminal holding portion 115b and is connected to the other end of the field coil of the automotive alternator.
However, the above-described conventional slip ring apparatus is disadvantageous in that the metallic leads 113 and 114 possibly deform during its manufacturing processes.
More specifically, the metallic leads 113 and 114 electrically connecting the slip rings 111 and 112 to the field coil are made of a thin metallic plate or a thin metallic wire. In the manufacturing process, the metallic leads 113 and 114 connected to the slip rings 111 and 112 are integrally molded in the resin or comparable insulating member 115. There is the possibility that the metallic leads 113 and 114 may deform undesirably due to inaccurate size of the leads, or defective connection between the leads and the slip rings, or an external force applied on the leads.
If such deformed metallic leads are placed in the molding dies, the deformed metallic leads will not stay as designated and may also damage the molding dies.
According to the slip ring apparatus 101 shown in FIG. 10, the first metallic lead 113 provides an electrical path to the first slip ring 111 located far from the field coil and underlies (i.e., extends radially inside with respect to) the second slip ring 112 located relatively close to the field coil. Thus, the first metallic lead 113 is electrically isolated from the second slip ring 112.
However, when the first metallic lead 113 deforms as shown in FIG. 11, there is the possibility that first metallic lead 113 may contact with the second slip ring 112. If the electrical connection or contact between the first metallic lead 113 and the second slip ring 112 is kept during the insert molding process, it will cause electrical short circuit between the first metallic lead 113 and the second slip ring 112 when this manufactured slip ring apparatus 101 is practically used because the fist and second slip rings 111 and 112 are brought into different electric potentials.
When electric short circuit occurs between the first metallic lead 113 and the second slip ring 112, the electric power generation will stop or heat generation will occur due to insufficient electric conduction. In the worst case, the insulating member 115 may burn out. Even when the first metallic lead 113 is covered by an insulating film, bringing the first metallic lead 113 into contact with the second slip ring 112 will possibly cause a damage of the insulating film. Thus, similar problem will arise.
If a non-destructive inspection using an X-ray or the like is performed to detect such a deformation of the first metallic lead 113 for each finished product of the slip ring apparatus, the second slip ring 112 (i.e., a metallic member surrounding the first metallic lead 113) will conceal the deformed portion of the first metallic lead 113 from the X-ray. Thus, using the non-destructive inspection is not substantially effective. This is the reason why the sampling inspection needs to be performed by a worker who randomly picks up a finished product, although relying on the sampling inspection cannot assure a stable manufacturing of high-quality slip ring apparatus.